Applicant claims, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Jun. 18, 1999 of a German patent application, copy attached, Serial Number 199 29 557.3, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The invention relates to a method and a circuit for adjusting a switching threshold of a touch probe.
2. Discussion of Related Art
From European Patent Disclosure EP 0 501 680 B1, a circuit for a touch probe (touch system) with a deflectable touch element is known that upon a deflection of the touch element out of its position of repose outputs a switching signal. Such touch probes are used for determining the position of workpieces that are chucked in machines, especially milling machines, that machine material.
The term position of repose of the touch element is understood to mean a position of the touch element in which it as yet has no contact with a workpiece. Only upon contact of the touch element with the workpiece is the touch element moved out of its position of repose.
To ascertain whether the touch element has been deflected out of its position of repose, a voltage is measured, which drops across contacts disposed in the touch probe. Upon deflection of the touch element out of its position of repose, the touch probe causes a change in the voltage dropping across the contacts. If the voltage exceeds a predetermined switching threshold, the touch probe outputs a switching signal.
A known touch probe also readjusts the switching threshold after a deflection of the touch element and its return to its position of repose. This readjustment is necessary because, as a result of material wear and the effect of moisture and temperature, the touch element after a deflection out of its position of repose does not always return to the position of repose it originally occupied; instead, it assumes a position that deviates slightly from the originally assumed position of repose, or in other words it assumes a new position of repose. If the switching threshold were not readjusted, the circuit would always generate a switching signal even though the touch element no longer has any contact with the workpiece. Thus, upon assumption of the new position of repose, the switching threshold is adjusted to a value that corresponds to the resistance or voltage dropping across the contacts at the new position of repose of the touch element.
However, it is a disadvantage of the known touch probe that a deflection of the touch element that occurs during the adjustment of the switching threshold is not detectable, since the adjustment of the switching threshold from one value to another is caused by the adoption of an instantaneous value. Slight variations in the measured voltage, which are caused by a slight deflection of the touch element, can therefore not be detected by the known touch probe during a simultaneous adjustment of the switching threshold. At the same time, vibration of the machine in which the touch probe is used can affect the known touch probe, causing an incorrect adjustment of the switching threshold and failing to assure proper function of the touch probe.
It is therefore an object of the present invention to disclose a method and a circuit that assure a more accurate, malfunction-free adjustment of the switching threshold of a touch probe (touch system).
The above object is attained according to one aspect of the present invention that regards a method for adjusting a switching threshold of a touch probe, which has a touch element. The method includes deflecting the touch element out of a position of repose and causing a defined change in a measurable system parameter based on the deflecting of the touch element. Adjusting a switching threshold to a value dependent on the value of the measurable system parameter at the position of repose of the touch element, wherein the switching threshold is determined from a reference value, which is variable in increments and which is compared with the value of the measurable system parameter at the position of repose of the touch element and the reference value is corrected by one increment at a time, based on the value of the measurable system parameter at the position of repose of the touch element, if the reference value deviates from the value of the measurable system parameter at the position of repose of the touch element.
The above object is also attained by a second aspect of the present invention that regards a circuit for adjusting a switching threshold of a touch probe, which has a touch element that can be deflected out of a position of repose and which upon a deflection of the touch element out of its position of repose causes a defined change in a measurable system parameter and outputs a switching signal when the value of the system parameter exceeds the switching threshold. The circuit includes a device for measuring the measurable system parameter, an electrical circuit for ascertaining a reference value from which the switching threshold is determinable and an incremental corrector that changes the value of the measurable reference value to the value of the measurable system parameter at the position of repose of the touch element, in which the switching threshold is determinable from the reference value.
Accordingly, in an embodiment of a method according to the present invention for adjusting a switching threshold of a touch probe, which has a touch element that can be deflected out of its position of repose and which upon a deflection of the touch element out of its position of repose effects a defined change in a measurable system parameter and outputs a switching signal if the system parameter exceeds the switching threshold, the switching threshold is determined from an incrementally variable reference value. This reference value is compared at predeterminable times with the value of the system parameter in the position of repose of the touch element and is corrected by one increment at a time on a basis of this value of the system parameter, if the reference value deviates from this value of the system parameter.
The term xe2x80x9cincrementally variable valuexe2x80x9d is understood to mean a value that can never be increased or decreased by more than a predeterminable value, namely one increment. In particular, the incrementally variable reference value is formed by a digital signal.
The term xe2x80x9ctouch probexe2x80x9d is understood to mean a touch system which as its essential components has both a touch element and a generator for generating a switching signal.
In an embodiment of a method according to the present invention, the adjustment of the switching threshold accordingly is accomplished by incremental, repeated slight approximation of the reference value to a value that is dependent on the value of the system parameter in the position of repose of the touch element. The invention has the advantage that on the one hand it assures precise adjustment of the switching threshold at any time. Secondly, it assures that as a rule even during the adjustment operation, a simultaneous deflection of the touch element is detectable.
In a first variant of an embodiment of a method according to the present invention, the reference value is corrected upon assumption of operation of the touch probe, that is, when the touch probe is switched on or a supply voltage is applied to the touch probe, only until such time as the reference value attains or exceeds the value of the system parameter in the position of repose of the touch element. Next, the touch probe is determined from the reference value. To that end, it is preferably provided that the reference value, upon attaining or exceeding the value of the system parameter in the position of repose of the touch element, is increased by a predeterminable value. The touch probe is then adjusted to this increased reference value.
The predeterminable, preferably constant value by which the reference value is increased is selected to be so slight that the switching threshold, set to the increased reference value, deviates only slightly from the value of the system parameter in the position of repose of the touch element. This increases the accuracy of the touch probe, so that even an only slight deflection of the touch element is detectable.
In a further version of an embodiment of a method according to the present invention, it is provided that the switching threshold is repeatedly readjusted as long as the touch probe is in operation. Thus, shifts in the position of repose of the touch element caused by wear, temperature and moisture during the entire duration of operation of the touch probe are compensated for, and proper functioning of the touch probe at all time is assured. The switching threshold is corrected on the basis of the value of the system parameter in the position of repose of the touch element in such a way that the switching threshold deviates only slightly from the value of the system parameter in the position of repose of the touch element. The switching threshold is always, however, adjusted in such a way that the proper function of the touch probe is assured.
An already-adjusted switching threshold is preferably always adjusted to a new value if the already-adjusted switching threshold deviates by a predeterminable amount from the value of the system parameter in the position of repose of the touch element. The predeterminable amount is preferably determined by the interval that must be present between the switching threshold and the value of the system parameter in the position of repose of the touch element if reliable functioning of the touch probe is to be assured.
The switching threshold, or the reference value determining the switching threshold, is always varied only slightly here, so that normally a deflection of the touch element out of its position of repose during the adjustment or readjustment of the switching threshold is detectable. The reason for this is that normally in a touch-sensing operation the value of the system parameter changes faster than the incremental correction of the switching threshold according to the invention.
Preferably, the readjustment of the switching threshold is never performed until the touch element, after a deflection, has returned to its position of repose. A return of the touch element to its position of repose is preferably assumed whenever the touch probe has not been moved further within a predeterminable period of time, such as one minute. As an alternative, the return of the touch element to its position of repose can be detected by suitable sensors. In this alternative version, the readjustment of the switching threshold for instance occurs only after a predetermined period of time after the detection of the return of the touch element to the position of repose by the sensors.
As the system parameter, a voltage that drops over a detector system is preferably measured; its magnitude is a measure for the motion of the touch element out of its position of repose. In the event that the voltage measured by the detector system exceeds the switching threshold because of a motion of the touch element out of its position of repose, a switching signal is generated by the touch probe.
In a further version of an embodiment of a method according to the present invention, a further signal is generated by the touch probe in the event that upon the adjustment of the switching threshold, a predeterminable limit value of the switching threshold is exceeded. The touch probe is then switched off immediately, or else an alarm signal is output to the evaluation unit. It is advantageous that a readjustment takes place only within a predetermined adjustment range, which for instance is defined by the allowable operating conditions of components employed.
A circuit is suitable for performing the above-described method. With regard to the advantages of the circuit of the invention, reference is made to the remarks made above. According to an embodiment of the present invention, the circuit has a device for measuring a system parameter in the position of repose of the touch element. Furthermore, an electric (for instance digital) circuit is provided for ascertaining a reference value from which the switching threshold can be determined. The circuit also incrementally corrects the reference value to the value of the system parameter in the position of repose of the touch element. The switching threshold is determined from the corrected reference value.